Droughts - Extreme Events: A Physical Reconstruction and Risk Assessment - page 39

theisotopic composition of the water from which they were precipitated. The
oxygen isotope composition of water bodies is usually due to changes in the ori-
gin of the water source, variations in evaporation, or a combination of the two.
The oxygen isotope composition of lake waters in arid regions is largely a func-
tion of evaporation rates (Hoelzmann et al ., 2000). Because 16 Oislighter than 18 O,
it is preferentially removed from lake and ocean waters during evaporation. The
16 Oisreturnedtothewaterbody by precipitation and stream runoff and these
in turn are a function of the prevailing climatic regime. Hence, the ratio of

16 O

to 18 Orecords the nature of that climatic regime. Levels of

18 Oareconcentrated
in the lake water during periods of high evaporation (Smith et al ., 1997) when
the return of water to the lake is low. As a consequence, concentrations of

18 O
tend to become higher with progressive evaporation if that water body is not
recharged with rainfall containing

18 Oinsediments and
fossils suggest episodes of enhanced evaporation in comparison to rainfall. Low
values of

16 O. High values of

18 Osuggest that evaporation was in balance with precipitation and
thebody of water was not being reduced in volume. Negative values of

18 Oare
an indicator of times of high water input into lakes and correspondingly lower
evaporation.
Water bodies that are depleted in

18 O can also indicate that the moisture
that fell in this period was isotopically depleted (Hoelzmann et al ., 2000). The

16 Oratio can be determined by the geographical position of a site rel-
ative to the distance and nature of terrain over which rain bearing air masses
have travelled. Air masses passing over continental land masses fractionate rain-
fall. Isotopically heavier water will precipitate first, leaving isotopically lighter
watertofall farther inland. This means that unless water is added to the air
masses further inland, the water that is entrained within air masses passing
over continental landmasses will often be heavily depleted in

18 O/

18 O.
Abell et al .(1996)examined the oxygen isotope chemistry of fossil mollus-
can shells in lakes of northwestern Sudan, Africa. The highly depleted

18 O
values for the early Holocene suggest that the lakes were filled at this time
by monsoonal rains. After approximately 5000 years BP,

18 O increased suggest-

18 O fluctuated significantly in these
lakes between approximately 8000 and 7000 years BP suggesting that precipita-
tion was variable during this interval. A short sharp decrease in precipitation
possibly associated with an extended drought episode occurred sometime after
7000 years BP.

ing drier conditions ensued. The levels of

18 Olevels decreased markedly after 7000 years BP indicative of
apronounced wet phase after which they increased sharply suggesting a period
of severe drought occurred. The return of lower

18 Olevels towards the present
suggests that present-day conditions are similar (wetter) to those that occurred
during the early Holocene in this region.